I'm Melissa Block. And as we've been reporting, NATO and world leaders are sharpening their criticism of Russia over its actions in Eastern Ukraine. And in Ukraine, residents of a strategic port city are working alongside government soldiers. They're building up the city's defenses against a widely anticipated assault. NPR's Soraya Sarhaddi Nelson is Mariupol and sent this report.

SORAYA SARHADDI NELSON, BYLINE: A handful of residents in this industrial seaport came with a backhoe and shovels to dig deep trenches next to a windy Ukrainian checkpoint called number 14. The checkpoint is about 30 miles from where pro-Russian separatist fighters are encamped.

UNIDENTIFIED MAN: (Foreign language spoken). (Laughing).

NELSON: Some of the sweaty volunteers take a cigarette and water break. One is a 19-year-old computer science major, Alexei. Like many people in Eastern Ukraine, he fears reprisals and refuses to give his last name.

ALEXEI: (Foreign language spoken).

NELSON: Alexei says, it's important to dig these holes for the soldiers who man this checkpoint, so they can hide from artillery fire and shelling and protect Mariupol. The soldiers embrace the public show of support. Their smiles and relaxed demeanor today are in stark contrast to the fatalism they expressed at this checkpoint earlier this week.

A witness to me then that the soldiers here had divided themselves into two groups, one of which planned to stay and fight to the death so the other could get away. Ukrainian captain Ruslan Muzichuk says, people view the secessionist movement as a dead end. He predicts no one is going to recognize the country called New Russia the rebels want to create. Khotlubey believes it will become a, quote, "territory of social outcast." But that doesn't mean he and others aren't taking the threat the advancing separatist force poses seriously.

If the separatists take Mariupol, than Russia will be able to create a land bridge to Crimea, which it forcibly took from Ukraine earlier this year. Like many Eastern Ukrainians, Mariupol volunteer Alexei has closely followed his government's accusations that Russia is stepping up its deployments of troops and armaments help the rebels. Kiev officials say, four military columns crossed in eastern Ukraine this week alone.

ALEXEI: (Through translator) Before, we were dealing with a group of bandits or riffraff. We could kick them out by ourselves. But this is no longer a war of opinions. This is a war against Russia.

NELSON: For their part, the rebels claim the weapons and military vehicles are all seized from Ukrainian forces and all the thousand-plus Russians fighting alongside them are volunteers. Soraya Sarhaddi Nelson, NPR News, Mariupol. Transcript provided by NPR, Copyright NPR.